October 16, 2013

Ancient Arthropod Had Massive Claws And Spider-Like Brain

While finding an intact fossil of any kind can be an exciting discovery, an international team of paleontologists recently discovered the earliest known complete nervous system in the fossil of a never-before described creature that crawled or swam in the ocean some 520 million years ago, according to a report in the journal Nature.

The researchers said the fossil belongs to an extinct group of marine arthropods known as megacheirans, which is Greek for "large claws,” and solves the mystery of where this group sits in the tree of life.

"We now know that the megacheirans had central nervous systems very similar to today's horseshoe crabs and scorpions," said study author Nicholas Strausfeld, a neuroscientist at the University of Arizona. "This means the ancestors of spiders and their kin lived side by side with the ancestors of crustaceans in the Lower Cambrian."

The scientists found the inch-long creature at a site in southwest China and identified it as a member of the extinct genus Alalcomenaeus. Like many other arthropods, these creatures had a drawn-out, segmented body with about a dozen pairs of appendages that allow the animal to swim, crawl or both. They also had long, scissor-like appendages attached to the head, possibly for grabbing or sensing objects.

A previous theory said the so-called ‘mega claw’ meant that megacheirans were related to chelicerates, because it has an “elbow joint” like that seen on the fangs of a spider or scorpion.

"However, this wasn't rock solid because others lined up the great appendage either a segment in front of spider fangs or one segment behind them," said study co-author Greg Edgecombe, an expert on invertebrates at the Natural History Museum in London.

"We have now managed to add direct evidence from which segment the brain sends nerves into the great appendage,” Edgecombe continued. “It's the second one, the same as in the fangs, or chelicerae. For the first time we can analyze how the segments of these fossil arthropods line up with each other the same way as we do with living species – using their nervous systems."

Using different imaging and image processing techniques, the study team was able to take advantage of iron deposits that had accumulated in the animal's nervous system during the fossilization process to create an outline of the animal’s entire nervous system.

After comparing the fossilized nervous system to nervous systems of horseshoe crabs and scorpions, the researchers concluded the 520 million-year-old Alalcomenaeus was a member of the chelicerates.

The researchers discovered biomarkers that are found in the brains of scorpions and spiders in their fossil, such as the three clusters of nerve cells fused together as a brain that were also connected with some of the animal's bodily nervous system. This system is much different from crustaceans where parts of the nervous system are further apart and arranged like the rungs of a rope ladder.

To confirm their findings, the researchers cross-referenced their findings with an existing catalog of about 150 evolutionary characteristics used to categorize arthropods.

"Greg plugged these characteristics into a computer-based cladistic analysis to ask, 'where does this fossil appear in a relational tree?'" Strausfeld said. "Our fossil of Alalcomenaeus came out with the modern chelicerates.

"The prominent appendages that gave the megacheirans their name were clearly used for grasping and holding and probably for sensory inputs,” Strausfeld added. “The parts of the brain that provide the wiring for where these large appendages arise are very large in this fossil. Based on their location, we can now say that the biting mouthparts in spiders and their relatives evolved from these appendages."